Gold alloys have been deposited for many years onto watchcases, watchbands, eyeglass frames, writing instruments, costume jewelry, and the like. The karat of these deposits usually ranges from 12 to 18, the deposit thicknesses range from 2 to 20 microns, and the deposit colors are pale yellow to pink. For many years, the most successful electroplated gold alloy for these applications has been gold-copper-cadmium. Since cadmium is such a poisonous metal, the industry has been searching for a substitute for cadmium which does not have its toxicity. In addition to being non-toxic, the gold alloy deposits produced with this cadmium substitute must have the required physical characteristics, as follows:
1. The deposits must have the correct color, as required. Usually, these colors are the Swiss standard "1-5N," which range from specific pale yellow to pink gold alloys with the "2N" yellow grade being preferred.
2. The deposits must be bright so that no further polishing is required after plating. This degree of brightness must be maintained even for thick deposits as high as 20 microns.
3. The plating bath must produce deposits that exhibit levelling such that tiny imperfections in the basis metal are smoothed out or covered.
4. The karat of the deposits should be as required. These karats generally range from about 12 to 18, or about 50-75% gold.
5. All deposits must be reasonably ductile and capable of passing the required ductility tests, even with thicknesses as high as 20 microns.
6. The deposits should be corrosion resistant and capable of passing the required corrosion tests.
Attempts have been made in the past to deposit gold-copper-zinc alloys as a substitute for the conventional gold-copper-cadmium alloys. For example, European Patent Application 03 04 315 Al discloses a process for depositing gold-copper-zinc alloys where each of the three metals is present in the plating bath as the cyanide complex. Bismuth and tellurium are additives disclosed for improved corrosion resistance of the deposits. Nothing is mentioned in the disclosure or the examples about thick deposits with respect to brightness or ductility or the ability to produce deposits free of cracks. The invention stresses improved corrosion resistance.
The German Offenlegungsschrift No. DE 36 33 529 A1 plates gold-copper-zinc alloys from plating baths in which the gold is present as a cyanide complex, zinc is present as the zinc chelate, and the plating bath is free from zinc cyanide complexes. The disclosure states in column 2, lines 22-25 that thicker deposits are brittle, cracked, and can exfoliate; which indicates that this bath is only suitable for electroplating thin deposits.
German Offenlegungsschrift No. DE 33 45 795 A discloses gold-copper-zinc electrodeposits from solutions that contain gold and copper as their alkali cyanide complexes, with zinc present as an alkali zinc chelate. The alkali metal is sodium instead of potassium. Deposits are bright but there is no discussion of brittleness in thicker deposits, and the deposit thickness in the example is relatively thin.
German Offenlegungsschrift No. DE 36 01 559 A discloses gold-copper-zinc electrodeposits from solutions that contain gold and copper as their alkali cyanide complexes and/or as an alkali zinc chelate. There is no discussion of brightness or brittleness in thick deposits.
German Offenlegungsschrift No. DE 30 20 765 A1 discloses copper-gold-zinc alloys in which all three metals are present in the plating bath as their cyanide complexes. The baths also must contain potassium carbonate or bicarbonate as additives. Although the disclosure states that deposits are ductile, no deposit thicknesses are stated.
None of the above disclosures have resulted in a commercially acceptable plating bath. None of them have been shown to be capable of producing deposits with the required characteristics given above. In all the above disclosures, attempts to produce gold-copper-zinc deposits by following the examples of these references were unsuccessful in that the resultant deposits were very brittle in the thickness range of 10 to 20 microns. In addition, most deposits exhibited spontaneous cracking or exfoliation, and brightness and levelling characteristics were far inferior to current industry requirements.